This invention relates to a three force balanced mechanism.
The three force balanced mechanism of this invention is utilized to detect such process quantities as temperature, pressure liquid level, flow quantity, etc., for converting them into air pressure signal, electric signal, etc., to transmit these converted signals to receivers, controller, for example, or to use the converted signals in the situation. Among prior art pneumatic instruments utilizing the three force balanced system, has been known a instrument constructed as shown in FIG. 1. Thus, the instrument shown in FIG. 1 comprises a detector A and a converter unit B. The detector A comprises a pressure vessel 1 with pressure receiving diaphragms 2 and 3 on both sides thereof. Liquid 4 such as a silicone oil is sealed in the pressure vessel 1. The lower end of a pressure derive out rod 5 is inserted into the pressure vessel 1 and the portion of the rod 5 where the rod 5 extends outwardly from the pressure vessel 1 is hermetically and swingably sealed to the vessel 1 by means of a well known torque tube 6. The inner end of the rod 5 is connected to the inner sides of the pressure receiving diaphragms 2 and 3 through a strap 7.
Consequently, where a high pressure P.sub.H and a low pressure P.sub.L are respectively applied to diaphragms 2 and 3 these diaphragms displace according to the difference between these high and low pressures. This displacement moves the sealed liquid 4 from the high pressure side to the low pressure side so that the pressure-derive-out-rod 5 would tilt about a pivot point O near the torque tube 6 whereby the pressure difference can be derived out.
The counter unit B has a floating pivot 11 and one end of an input force transmitting member 10 is connected to the floating pivot 11, the other end of the input force transmitting member 10 being connected to the pressure-derive-out-rod 5. One end of a power transmitting member 12 comprising feedback force transmitting means is also connected to the floating pivot 11, the power transmitting member 12 extending in a direction at right angles with respect to the input force transmitting member 10. The other end of the power transmitting member 12 is connected to one end of a feedback beam 14 with an intermediate point thereof swingably supported by a fulcrum 13. Near the other end of the feedback beam 14 are provided a nozzle 15, a zero point adjusting spring 16 and a feedback bellows 17. The nozzle 15 opposes the lower end of the feedback beam 14 with a small gap the back pressure generated by the nozzle 15 is amplified by the relay valve and is led to the feedback bellows 17 so as to normally maintain the feedback beam 14 in a balanced state with the pressure-derive-out-rod 5. One end of a span setting wire 18 extending in a direction different from that of the force transmitting member 12 is also connected to the floating pivot 11, and the other end of the wire 18 is held in an arcuate groove 19 about the pivot 11.
Accordingly, in the converter unit B, a vector mechanism is formed by beams about the floating pivot 11, which has a computing performance based on the principle of trigonometric functions. More particularly, when a force F.sub.1, caused by the output of the detector A and tending to incline the outer end of the pressure-derive-out-rod 5, is applied to the floating pivot 11 through the input force transmitting member 10 to press the pivot 11 to the right, at portions of the span setting wire 18 and the force transmitting member 12 near the pivot 11 reaction components F.sub.2 and F.sub.3 of the force F.sub.1 and created in the centirifugal directions.
The component F.sub.3 is given by an equation F.sub.3 =F.sub.1 .multidot.tan .theta. which acts to raise the righthand end of the feedback beam 14 at the lower end of the motion transmitting member 12 so as to rotate the feedback beam 14 in the counterclockwise direction about the fulcrum 13, thus decreasing the gap between the feedback beam 14 and nozzle 15. As a consequence, the back pressure of nozzle increases to obtain an output P out corresponding to force F.sub.1 which is transmitted to outside as an air pressure signal through a tube 23 and to the feedback bellows 17. Consequently, the feedback beam 14 rotates in the clockwise direction about fulcrum 13 to balance with the component F.sub.3, that is the force F.sub.1 applied by the pressure-derive-out-rod 5.
When the outer end of the span setting wire 18 is moved along the arcuate groove 19 to vary angle .theta. between the wire 18 and the input force transmitting member 10, the components F.sub.2 and F.sub.3 vary also corresponding to the variation in angle .theta., with the result that the gain is varied to vary the measuring range.
With such prior art three force balanced mechanism, since a spring support is utilized for the purpose of decreasing the sliding friction loss at the joint between the span setting wire 18 and the floating pivot 11, from the standpoint of construction it is impossible to greatly vary the set angle .theta. of the span setting wire 18 (usually, the limit is about 10.degree.), for this reason, there is a defect that the measuring range is narrow. To obviate this defect it has been proposed to construct the feedback bellows 17 as a double wall construction and to change in two stages the feedback force by means of a range transfer switch 24 so as to increase the measuring range. Such proposal, however, requires inner and outer bellows 17A and 17B and the range transfer switch 24 so that the construction is complicated, manufacturing cost is high, and a large zero point shift is resulted at the time of switching the range.